VBoxManage controlvm        <uuid>|<name>
                            pause|resume|reset|poweroff|savestate|
                            acpipowerbutton|acpisleepbutton|

#!/bin/bash
if [ -z "$1" ]; then
        echo "Usage: $0 VMNAME_or_UUID"
        exit 1
fi
set -x
VBoxManage controlvm  "$1" poweroff  #enforce turnoff
VBoxManage snapshot   "$1" restorecurrent   #retore state
VBoxManage showvminfo "$1" | grep State   #display state to ensure
VBoxHeadless -s       "$1"  #run in headless mode in background

[Unit]
Description=VBox Virtual Machine %i Service
Requires=systemd-modules-load.service
After=systemd-modules-load.service

[Service]
User=user
Group=vboxusers
ExecStart=/usr/bin/VBoxHeadless -s %i
ExecStop=/usr/bin/VBoxManage controlvm %i savestate

[Install]
WantedBy=multi-user.target

$ sudo systemctl enable vboxvmservice@vm_name.service

...
start(){
    su -c "VBoxHeadless --startvm thevm" -s /bin/bash theuser &
    # maybe another vbox command
}

stop(){
    su -c "VBoxManage controlvm thevm acpipowerbutton" -s /bin/bash theuser
    # maybe another vbox command
    while [ "`su -c 'VBoxManage list runningvms' -s /bin/bash theuser`" != "" ]
    do
        echo waiting for VMs to shutdown
        sleep 3
    done
}
...

VBoxManage startvm VMNAME --type headless

VBoxManage controlvm VMNAME savestate

VBoxManage list runningvms

VBoxManage list runningvms | tr -s '" {' '%{' | cut -d '%' -f3  | while read uuid; do
   VBoxManage controlvm $uuid savestate;
done